One of the main antiviral defense mechanisms in plants, and also in animals, is based on the phenomenon of PTGS (post-transcriptional gene silencing) which results in the specific degradation of foreign or overexpressed RNAs.
In the case of viruses, the viral genome is the triggering agent and the target of the phenomenon.
Concomitantly, viruses have developed functions which allow them to bypass this type of defense mechanism, by specializing some of their proteins in silencing suppressor functions which target key steps of PTGS.
Many proteins, referred to as “silencing suppressors” have been described in phytopathogenic viruses by virtue of non-host interactions for some of them in Nicotiana benthamiana. In fact, PVX-potential suppressor protein vectors have been generated and inoculated (by agroinfiltration) onto N. benthamiana plants exhibiting PTGS in order to evaluate the suppressor capacity of the proteins studied. The P1 protein of the rice yellow mottle sobemovirus (hereinafter referred to as “RYMV”), a virus exclusively restricted to the infection of rice and some wild-type grasses, has thus been described as a silencing suppressor.
The P1 protein of RYMV is a protein of 158 amino acids encoded by the first of the 4 viral ORFs. This protein is translated during the entire infectious cycle and detected in the form of a doublet of 18 and 19 kDa. It exerts an indirect action on viral replication and appears to be necessary for the movement of RYMV from cell to cell and for long distance movement. Complementation studies have shown that it acts in trans during viral infection. This protein has been described as a silencing suppressor that acts in the “nonautonomous” cell mode and therefore on the propagation of the PTGS signal.
The inventors have evaluated the effectiveness of the P1 protein and of RYMV in suppressing PTGS in a context of natural interaction between RYMV and its host, rice.
The studies carried out on a collection of RYMV isolates, characterized in terms of pathology and phylogeny, have made it possible to demonstrate P1 diversity and functional differences in activity which are related to this diversity. These studies have also made it possible to explore the quantitative and qualitative effects of these P1s and RYMV on PTGS suppression.